The invention relates to an optical character generator for an electro photographic printer for generating a charge image on a revolving charge storage drum, the charge image being in the form of micro-image lines generated continuously during in each case one printing cycle. The generator has a plurality of modules arranged in the form of rows, consisting of plurality of light-emitting diode chips having in each case light-emitting diode chips having in each case light-emitting diodes which are in each case arranged row by row underneath one another and which are also offset with respect to one another, of driver chips associated with the latter on both sides and of drive chips connected to the latter for buffer storage and conversion of printing information, serially supplied by a printer control via a bus system, into individual drive signals for the drive chips for selectively switching-on the light-emitting diodes.
Printers operating in accordance with the principle of electrophotography contain an optical character generator. This character generator is used for converting an item of printing information present in the form of electronic data into an optical image by means of which a photoconductive layer, for example a continuously circulating charge storage drum of the printer, is then exposed in order to generate a latent charge image. This charge image is developed in familiar manner and reprinted, for example, on paper.
For this application, optical character generators are of advantage which expose the charge storage drum row-by-row in its full length since the exposure process can then be carried out without any mechanical movement. Such a row-by-row configuration of the optical character generator requires a separate light source for each imaging point within the row. Light-emitting diodes are preferably used as light sources since these components offer the possibility of a very compact arrangement.
The aim of such printers is to achieve a high quality of the printed image. The prerequisite for this is a corresponding resolution, 240 dpi (=dots per inch) and more not being unusual at all for electro photographic printers. An optical character generator constructed in rows for an electro photographic printer therefore contains a plurality of light-emitting diodes, frequently several 1000. The drive system for the light-emitting diodes, which must be individually switched in dependence on the printing information supplied to the optical character generator, is correspondingly complex. From IEEE Transaction on Consumer Electronics, Vol. CE-32, No. 1, February 1986, pages 26 et seq., an optical character generator is known in which a monolithically constructed row of light-emitting diodes is driven via a plurality of drive chips in the form of integrated circuits arranged on both sides of this row of light-emitting diodes. This known optical character generator is designed for applications in electro photographic printers with a printing capacity of about one DIN A4 sheet per second which corresponds to a line frequency of about 3 kHz. This requires a data rate of about 8 Mbit/s, resulting in a data clock frequency of 1 MHz with a total of eight parallel data channels.
Apart from data registers, the drive chips contain an integrated driver circuit comprising a plurality of driver stages which are in each case allocated to one of the light-emitting diodes. The driver circuit has a current source which can be calibrated to the chip and to which the driver stages are connected in parallel. The driver stages are constructed in such a manner that the driver current flowing via the individual stage either activates the associated light-emitting diode or flows away via a ground connection. It is considered to be an advantage of this solution that the entire current loading in the optical character generator remains constant with time independently of the respective printed image.
However, it must be realized that quite considerable currents flow with several 1000 light-emitting diodes and a corresponding number of driver stages in the optical character generator and the total thermal loading is therefore very high with the compact type of construction. Although this thermal loading is kept constant in the main in the known solution, this advantage is gained at the cost of a not inconsiderable proportion of the currents supplied by the current sources being uselessly drained directly via ground which generates further heat which is avoidable per se.
In addition, all driver stages of the known solution are jointly activated via a strobe signal and kept in the active state for a period of this signal. It is known, however, that the light yields of light-emitting diodes are subject to tolerances which means that the radiation energy emitted during a predetermined period of time is an individual value for each light-emitting diode. It follows from this that the radiation energy impinging in image dots on the charge storage drum is different which has a disadvantageous influence on the print quality.
Finally, a multiple of the said line frequency is required for high-speed printers operating in accordance with the electro photographic principle and it should preferably also be possible to process various paper formats. An optical character generator having a line length which corresponds to the transverse format of a DIN A4 page is not adequate for this. In order to print papers in the DIN A3 format, the optical character generator must have at least a line length which corresponds to the length of one longitudinal side of a DIN A4 sheet. Given such constraints, however, the optical character generator can no longer be constructed in accordance with the teachings of the known solution.
From U.S. Pat. No. 4,571,602, a modular optical character generator for an electro photographic line printer is also known the modules of which in each case contain several light-emitting diode chips and the associated drive circuit. To achieve a corresponding resolution of the micro-image dots, the light-emitting diodes are arranged in two rows underneath one another. In this arrangement, the light-emitting diodes of one row are in each case offset with respect to those of the other row by one centre-to-centre distance in the line direction. Thus, the problem exists of synchronizing the drive of the light-emitting diodes of the two light-emitting diode rows which are geometrically offset with respect to one another transversely to the row direction, even though the printing information corresponding to all image dots of a micro-image line is transmitted serially bit by bit in one printing cycle by a printer control.
To solve this problem, each light-emitting diode row is associated with its own shift register chain in which the printing information transferred is stored bit by bit and in cyclic alternation. After the transmission process, one shift register chain contains the printing information for all even-numbered image dots and the other shift register chain contains the printing information for all odd-numbered image dots of one and the same micro-image line. During the subsequent printing process, the printing information is read out in parallel from one of the two shift register chains and transferred directly into driver chips for the light-emitting diodes of one diode row so that by this means, for example, all light-emitting diodes associated with even-numbered image dots are activated simultaneously but individually.
At the same time, the light-emitting diodes of the other diode row associated with the odd-numbered image dots are activated in the same manner. Taking into consideration the geometric offset of the two diode rows with respect to one another, however, the light-emitting diodes of the second diode row are driven on the basis of an item of printing information for a micro-image line the even-numbered image dots of which were already generated a predetermined number of printing cycles before. For this purpose, delay circuits are inserted between the second shift register chain and the driver circuits for the second diode row. In these delay circuits, the transport of information is controlled by a clock signal synchronizing the printing cycle, in such a manner that the printing process occurs at the same time for even-numbered and odd-numbered image dots of two different image lines.
Due to the plurality of driven light-emitting diodes within a short period, this results in a considerable impulsive current loading of the optical character generator in the form of the sum of the driver currents occurring. The mean current loading is proportional to the line length of the optical character generator and inversely proportional to the spacing of the micro-image dots.
Finally, GB-A-2 099 221 describes for such an optical character generator in particular the technology which makes it possible inexpensively and operationally reliably to construct densely packed modules of LED chips and drive and driver chips. Solutions for combating the problem of average current loading in optical character generators with high packing density are not addressed in this context.